Organic EL devices are known. Such an organic EL device is a self-emitting device applicable to illuminators, displays and the like, and thus attracting more and more attentions.
One of known organic EL devices includes a plurality of emitting layers that each emit light of a different wavelength. Mixture of the light emitted by the emitting layers provides mixed-color light.
An example of such organic EL devices includes a layered red-emitting layer, green-emitting layer and blue-emitting layer, and provides white light in which emission from the emitting layers are mixed together (see, e.g., Patent Documents 1, 2 and 3, Non-Patent Document 1).
However, such a known organic EL device as disclosed in Patent Document 1 requires at least three emitting layers to be layered, which has led to complication of the manufacturing process and increase in cost.
One possible solution is to structure a single emitting layer to contain dopants for emitting three colors of red, green and blue, so that emission of the dopants as a whole will provide white emission.
Such a structure can unify the conventional three-separate emitting layers into a single emitting layer, and realize simplification of the manufacturing process and reduction in cost.
However, it has been difficult to adopt such a structure because of the following problems.
In terms of singlet energy gaps of dopants for fluorescent emission, a dopant for emission of longer wavelength (redder emission) has a smaller singlet energy gap, and a dopant for emission of shorter wavelength (bluer emission) has a greater singlet energy gap.
Accordingly, the singlet energy of blue to green dopants tends to transfer to a red dopant, so that the blue to green emission is difficult to be obtained.
Hence, only the red dopant provides intense emission while blue to green emission is difficult to be obtained, so that the device as a whole will provide reddish emission.
Examples of methods for preventing the above-described problems are: a method of balancing three color emission by suppressing the energy transfer between the color dopants (in particular, the energy transfer to the red dopant) by wholly reducing the doping concentrations of the dopants; and a method of relatively weakening the red emission by reducing the doping concentration of the easily-emitting red dopant to be smaller than those of the other dopants.
However, these methods require minute adjustment of the doping concentrations, which would bring considerable difficulty to the manufacture of devices.
Although the problems are described above by exemplifying the fluorescent dopants that utilize the singlet energy, the same problems will be encountered when phosphorescent dopants utilizing triplet energy are used.
Further, such a conventional organic EL device as disclosed in Patent Document 1 provides phosphorescent emission by transferring the triplet energy from a fluorescent host of a fluorescent-emitting layer to a phosphorescent dopant of a phosphorescent-emitting layer. However, in order to secure the transfer of the triplet energy, the fluorescent-emitting layer is required to be thinned, which has led to shortening of the device lifetime.    Patent Document 1: US2002/182441    Patent Document 2: WO2006/038020    Patent Document 3: WO2004/060026    Non-Patent Document 1: nature vol 440 p. 908